linux-kernel - Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore

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Date:   Sat, 29 Feb 2020 15:36:44 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for
 __fpu__restore_sig()

On Fri, Feb 28, 2020 at 02:13:29PM -0800, Yu-cheng Yu wrote:
> If the XSAVES buffer already has current data (i.e. TIF_NEED_FPU_LOAD is
> set), then skip copy_xregs_to_kernel().  This happens when the task was
> context-switched out and has not returned to user-mode.

So I got tired of this peacemeal game back'n'forth and went and did your
work for ya.

First of all, on my fairly new KBL test box, the context size is almost
a kB:

[    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
[    0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
[    0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
[    0.000000] x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registers'
[    0.000000] x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'
[    0.000000] x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
[    0.000000] x86/fpu: xstate_offset[3]:  832, xstate_sizes[3]:   64
[    0.000000] x86/fpu: xstate_offset[4]:  896, xstate_sizes[4]:   64
[    0.000000] x86/fpu: Enabled xstate features 0x1f, context size is 960 bytes, using 'compacted' format.

Then, I added this ontop of your patchset:

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 0d3e06a772b0..2e57b8d79c0e 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -337,6 +337,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
         */
        fpregs_lock();
        if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
+               trace_printk("!NEED_FPU_LOAD, size: %d, supervisor: 0x%llx\n",
+                            size, xfeatures_mask_supervisor());
                if (xfeatures_mask_supervisor())
                        copy_xregs_to_kernel(&fpu->state.xsave);
                set_thread_flag(TIF_NEED_FPU_LOAD);

and traced a fairly boring kernel build workload where the kernel
.config is not even a distro one but a tailored for this machine.

Which means, it took 3m35.058s to build and the trace buffer had 53973
entries like this one:

bash-1211  [002] ...1   648.238585: __fpu__restore_sig: !NEED_FPU_LOAD, size: 1092, supervisor: 0x0

which means I have

53973 / (3*60 + 35) =~ 251 XSAVES invocations per second!

And this only during this single workload - I don't even wanna imagine
what that number would be if it were a huge, overloaded box with a
signal heavy workload.

And all this overhead to save 16 + 24 bytes supervisor states and throw
away the rest up to 960 bytes each time.

Err, I don't think so.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette